1. Field of the Invention
This invention relates to a method and apparatus for interfacing an ion mobility spectrometer (IMS) to a mass spectrometer (MS) that utilizes collisional focusing. More specifically this invention is related to the internal modification of a standard configuration IMS and MS instrument, which will allow the two instruments to be joined in tandem and thereby achieve a greater selectivity and sensitivity of analysis over a single sample.
2. Background of the Invention
The use of improvised explosive devices (IEDs) for terrorist purposes will continue to be a threat to public safety for the foreseeable future. To help mitigate this threat, more than 15,000 explosive trace detectors (ETDs) are deployed at security checkpoints worldwide. The function of an ETD is to detect the presence of an explosive residue on a subject that has been in contact with an IED. Accordingly, the primary characteristics required of an ETD are selectivity and sensitivity.
Ion mobility spectrometry (IMS) is the analytical method most commonly implemented in ETD equipment. In IMS, the velocity of an ion though a buffer gas in the presence of an electric field is measured. Because the velocity of the ion is proportional to its reduced mass and cross-section, the ion is identified on the basis of its size. Moreover, because the ion is formed by efficient atmospheric pressure chemical ionization (APCI) processes, low limits of detection are observed in positive polarity for molecules with high proton affinities and in negative polarity for molecules with high electro-negativities. Consequent to these properties, IMS equipment that is operated in positive and negative polarity with ammonia and chlorine based reactant reagents has routinely demonstrated nanogram or better sensitivities for the detection of a suite of peroxide, nitro, and nitrate containing explosives.
While IMS has the selectivity and sensitivity needed to accurately detect trace amounts of explosives under a variety of conditions, opportunities for improvement exist regarding false positive and false negative responses. A false positive is typically attributed to the inability of IMS to differentiate an analyte molecule from an interferent molecule of a similar size, whereas, a false negative is typically attributed to the inability of APCI to ionize an analyte molecule in a competitive ionization environment. Further complicating the false negative rate is the constantly evolving threat matrix to which IMS must continuously adapt.
The most practical approach to decreasing the false response rates of IMS equipment is to modify the operating parameters of the instrument, such as buffer gas temperature, reactant reagent, electric field strengths, and detection algorithms, with the objective of increasing selectivity. These types of modifications are most effectively made using a detailed understanding of the gas phase ion chemistry that is intrinsic to the instrument being modified.
A detailed study of the ion chemistry of commercial. IMS based ETD equipment is not available in the literature.
For example, the ion species that are formed in these IMS devices during the analysis of typically encountered explosives and chemical interferents have not been identified with the certainty of mass spectrometry (MS). While information about sample and atmosphere composition and an understanding of gas phase ionization processes have provided informed estimates about the identities of these ions, the complexities of the reactions that occur at atmospheric pressure may lead to possible misidentifications when these types of assumptions are used.
For this reason, the most effective way to identify the ions in an IMS instrument with a high level of confidence is through direct mass measurements using a mass spectrometer, preferably a Tandem MS.
Although single unit IMS-MS combination instrumentation have been used in the field of ETD, such instrumentation is extremely expensive and produced in very limited quantity. The IMS/MS interface configuration of the invention described herein provides a practical apparatus and method for the interface of an easily obtainable COTS IMS and COTS MS detection system.